Multilayer structures in which a gas barrier layer containing aluminum or aluminum oxide as a component is formed on a plastic film have been conventionally well-known. In many cases, such a gas barrier layer is formed on a plastic film by a dry process such as physical vapor deposition (PVD) or chemical vapor deposition (CVD). Such a multilayer structure is used as a packaging material for protecting an article (such as a food) which is susceptible to quality change induced by oxygen. The multilayer structure is used also as a component of a protective sheet for an electronic device to preserve the characteristics of the electronic device when the electronic device is required to have gas barrier properties and water vapor barrier properties.
For example, aluminum-deposited films have light shielding properties as well as gas barrier properties and are typically used as packaging materials for dry foods.
Aluminum oxide-deposited films, which have transparency, are characterized by allowing visual recognition of contained substances and by enabling check for foreign matters with a metal detector and heating with a microwave oven. These films are thus used as packaging materials in a wide variety of applications such as retort food packaging.
For example, Patent Literature 1 discloses a multilayer structure having such a gas barrier layer containing aluminum, the multilayer structure having a transparent gas barrier layer composed of a reaction product of aluminum oxide particles and a phosphorus compound. Patent Literature 1 discloses a method for forming the gas barrier layer, in which a coating liquid containing aluminum oxide particles and a phosphorus compound is applied onto a plastic film, then dried and heat-treated.
Such conventional multilayer structures having a gas barrier layer have good initial gas barrier properties; however, when they are exposed to physical stresses such as deformation and impact, the gas barrier layer may suffer from defects such as cracks and pinholes which lead to deterioration in gas barrier properties.
Under such circumstances, Patent Literature 2 has proposed a multilayer structure that not only has good gas barrier properties but also is capable of maintaining the gas barrier properties at a high level even when exposed to physical stresses such as deformation and impact.
However, in some cases where the present inventors used the multilayer structures of Patent Literature 1 and Patent Literature 2 as packaging materials for retort foods, the multilayer structures experienced a decrease in interlayer adhesion and suffered from appearance defects such as delamination after retorting. In addition, in some cases where the present inventors used the multilayer structures of Patent Literature 1 and Patent Literature 2 in electronic devices, the multilayer structures suffered from delamination after a damp heat test.
Thus, a gas-barrier multilayer structure that maintains good properties even after retorting has been demanded. There has also been a demand for an electronic device including a multilayer structure that exhibits good interlayer adhesion and good barrier properties even at high temperature and high humidity.